Some obscure group of Protoss built themselves a super-ship capable of ripping whole fleets into metal bits. They intend to use this in some deranged quest to eliminate anything that is not "enlightened", as in anything that isn't them. This means Zerg, Terrans, even other Protoss.

Zerg Overmind begins whispering things into Ghosts's who patrol the place's minds, and eventually corrupt them, making them attack their brothers, thinking they are Zerg Hyrdralisks, the Zerg Overmind breaks loose, and now has a group of Ghosts escorting him. You are a memeber of the Protoss/Terran alliance task force sent out to destroy the Zerg Overmind.

Zerg Overmind begins whispering things into Ghosts's who patrol the place's minds, and eventually corrupt them, making them attack their brothers, thinking they are Zerg Hyrdralisks, the Zerg Overmind breaks loose, and now has a group of Ghosts escorting him. You are a memeber of the Protoss/Terran alliance task force sent out to destroy the Zerg Overmind.